Cancer is a major threat to human and non-human animal health, leading to reduced quality of life and, in too many cases, death. The burden placed on national, regional and local healthcare organizations to treat and prevent the various forms of cancer is significant in terms of the resources and manpower required. One of the main weapons vertebrates, including humans, have to combat disease is a functioning immune system. A brief consideration of immunotherapies to treat or prevent cancer might lead one to conclude that the effort held out little hope of success because immune systems guard against foreign, or non-self, materials and cancer cells arise from within, i.e., they are self materials. Continued progress in our understanding of cancer and immunology is modifying that view, however.
Mutant antigens are powerful targets for tumor destruction, e.g., in mice, and tumor-infiltrating lymphocytes targeting these mutations cause durable tumor regression in patients. Nevertheless, non-mutant antigens have been presumed by many scientists to be cancer-specific or “relatively cancer-specific” and safe antigens for vaccine approaches. However, adoptively transferred T cells can be orders of magnitude more effective and destructive than vaccinations. As a result, targeting MAGE-A3, HER-2 or CEA with T cells has caused death or serious toxicity in clinical trials now halted (8-11). As was shown in 2002, cancer cells with extremely high or very low expression levels of a target antigen differ only in the induction of immune responses, but not at the effector phase (15).
A publication in 1995 (6) established that somatic tumor-specific mutations resulting in mutant peptides are the cause of unique antigens, which are recognized by tumor-specific T cells. This was subsequently confirmed by many independent laboratories in studies on human and mice (e.g., 23-25). There it was shown that the unique immunodominant antigen on the UV-induced tumor 8101 was caused by a single base-pair substitution in the p68 oncogenic RNA helicase, a critical microRNA regulator protein (26-28).
Non-mutant antigens can nevertheless be cancer-specific antigens and safe targets for adoptive T cell transfer, and this realization involves a shift in focus from previous work caused by the discovery that Tn-O-glycopeptides occur as cancer-specific antigens, as disclosed in Science in 2006 (16). Tn antigen (1, 2) is expressed by a majority of common cancers of diverse origin and it is one of the earliest antigens identified on human tumors (FIG. 4) (18-20). Importantly, the peptide sequence is not part of the Tn antigen and not recognized by anti-Tn antibodies (for review see (12)). Antibodies that specifically bind only Tn are usually IgM and of limited use, i.e., for histochemistry but probably not CARs (Table 2). Occasional IgG-class anti-Tn antibodies are of poor specificity and affinity, and may slightly delay the outgrowth of Tn-expressing transplanted cancer cells when used in animals (54, 55).
It is likely that about 70-90% of common human cancers, such as breast, colon, prostate, ovary, lung, bladder and cervix cancers, express Tn (12). Conflicting data on the magnitude of expression of Tn on human tumors (56) can be largely explained by differences in affinities of the large number of different antibodies that have been experimentally produced most of them of very poor quality (with very few exceptions such as the IgM 5F4). Apparently, it is difficult for the epitope binding site of antibodies to bind the single sugar molecule with high affinity and specificity. While TF antigen (FIG. 1) is an oncofetal antigen highly expressed in the embryo and fetus (57), there is less evidence that Tn is also an oncofetal antigen (12), even though Tn antigens have been reported to be expressed perinatally in the brain but rapidly declining after birth (58). Most adults naturally have anti-Tn as well as anti-TF antibodies, probably due to antigenic stimulation by Tn and TF antigens expressed on the bacterial flora (13, 14); Tn antigen is also expressed on HIV-1 and pathogenic parasites (12).
Even though Tn was discovered by Dausset half a century ago (2) and Tn-expression on cancer cells over 40 years ago (18-21), technological advances that allowed the sophistication and rapid expansion of glyco-chemistry and glycobiology were only made in the last decade. There are still huge defects in our understanding of this field. As a further point on specificity, there is longstanding evidence for tolerance to many cancer testis antigens, HER-2 and CEA, indicating their expression on normal tissues and ultimately absence of true cancer specificity. By contrast, Tn-O-glycopeptides consistently have given the opposite result.
Most human cancers lack specific antigens that are predictably present and serve as effective targets for eradication by T cells. Every cancer cell type harbors a unique set of mutations causing different tumor-specific antigens. Identifying an effective unique antigen and isolating an appropriate TCR for transduction of autologous T cells for adoptive immunotherapy is still difficult despite the enormous technological progress being made. Adoptive immunotherapy using antibodies or T cells is clinically as well as experimentally the most effective immunotherapy, at least when clinically relevant cancers are considered (22). The remarkable success of adoptive immunotherapy with chimeric antibody receptors (CARs) and bispecific T cell engaging proteins (BiTEs) is, however, largely restricted to those specific for CD19/CD20-eradicating B cell malignancies and normal B cells in patients, i.e., hematopoietic cancers. Thus, there is a need to identify shared, yet tumor-specific, antigens on a wide range of solid tumors, and a concomitant need to develop prophylactics and therapeutics that can diagnose, prevent, treat or ameliorate a symptom of these cancers, along with methods for diagnosing, preventing and treating various cancers.